Crude oil development and production can include up to three distinct phases: primary, secondary, and tertiary (or enhanced) recovery. During primary recovery, the natural pressure of the reservoir or gravity drives oil into the wellbore, combined with artificial lift techniques (such as pumps) which bring the oil to the surface. But only about 10 percent of a reservoir's original oil in place is typically produced during primary recovery. Secondary recovery techniques extend a field's productive life generally by injecting water or a gas to displace oil and drive it to a production wellbore, resulting in the recovery of 20 to 40 percent of the original oil in place.
Enhanced oil recovery, or EOR, is a generic term encompassing techniques for increasing the amount of crude oil that can be extracted from a subterranean formation such as an oil field. EOR techniques offer prospects for ultimately producing 30% to 60% or more of the reservoir's original oil in place. Of these techniques, polymer flooding is particularly favored. Polymer flooding is generally accomplished by dissolving the selected polymer in water, and injecting the polymer solution into the reservoir.
However, since the target concentration of polymer in the solutions is typically about 10,000 ppm (1 wt %) or less, transport at the target concentration is not economically efficient. Transporting the dried polymers, while economically efficient, is sometimes not favorable for field use due to difficulties in fully hydrating the dry polymers in the field. To address these issues, various formulations have been developed to allow economically feasible transportation and storage. Specialized methods have also been developed to convert the formulations to use concentrations of fully hydrated polymers in the field.
Organic polymers traditionally used in EOR include water soluble polymers such as polyacrylamides, polyacrylates, copolymers thereof and copolymers of these with acrylamidomethylpropane sulfonic acid, ammonium functional monomers such as DADMAC (N,N′-diallyl-N,N′-dimethylammonium chloride), as well as hydrophobically modified versions of these, also called associative polymers or associative thickeners. Associative thickeners typically include about 1 mole % or less of a hydrophobic monomer such as a C8-C16 linear or branched ester of acrylic acid or N-alkyl adduct of acrylamide. The most commonly employed polymer for EOR is a copolymer of 70 mole % acrylamide and 30 mole % acrylic acid.
The EOR polymers are deliverable as powder, as a concentrate such as a 20 wt % polyacrylamide gel, or in the water phase of a water-in-oil (w/o) latex. Of these formats, water-in-oil lattices have the advantage of being deliverable in a liquid format that is easily handled in the field because the latex viscosity is lower than that of a water solution of comparable wt % polymer. Typically, such lattices include about 10 wt % to 80 wt % polymer solids, yet have a latex viscosity of less than about 2000 cP. Latex polymers are favored for use in offshore applications and other relatively isolated operations due to the ease of use and relatively simple equipment requirements.
Commercial w/o lattices are formulated for EOR by dissolving monomer in a high-solids aqueous solution to form a water phase, mixing a hydrocarbon solvent and a surfactant having a hydrophilic-lipophilic balance (HLB) of about 2 to 8 to form an oil phase, mixing the two phases using techniques that result in a water-in-oil emulsion or latex, and polymerizing the monomer via a free-radical azo or redox mechanisms. After polymerization is complete, a higher HLB surfactant (HLB>8) is often added as a destabilizer to facilitate latex inversion when water is added. “Inversion” is a term of art in EOR to describe the dilution of w/o lattices with a water source, causing destabilization of the latex and subsequent dissolution of the concentrated polymer particles to full hydrodynamic volume and maximum solution viscosity.
In EOR applications, it is a goal of field operators to achieve continuous inversion and hydration of w/o lattices to reach the target polymer solution concentration before the injection mixture reaches the reservoir rockface. In offshore EOR applications, the transit time between the topside mixing of the polymer with the injection water and the injection into the reservoir rock can range from about 5 minutes to about 180 minutes. In such applications, the final target concentration of the polymer solution is about 500 to 10,000 ppm (0.05 wt % to 1 wt %) in a pipe in line. However, inversion of conventional lattices at concentrations below 1 wt % is problematic. There exists a concentration effect in which w/o latex polymers invert more efficiently at target concentrations of about 1 wt % polymer or more. This is especially true in high temperature condition, high total dissolved solids conditions, or in both such conditions. When a typical anionic latex polymer is inverted at 1000 ppm in tapwater, for example, full solution viscosity cannot be reached even after several hours of stirring in the laboratory. Actual industrial conditions are much less favorable for reaching target concentrations of 1 wt % or less of fully inverted and hydrated polymer solutions in a 5-180 minute time frame.
Further, there is increasingly the need to address polymer flooding in challenging conditions encountered in reservoirs wherein ambient or produced water source contacted by the polymer includes high total dissolved solids, such as total dissolved solids of up to about 30 wt %. Another need is to address reservoirs where the available water source is present at an elevated temperature, such as 60° C. to 200° C. In some cases, the ambient or produced water source is both high total solids and is present at a high temperature. Field operators strongly prefer to use ambient or produced water sources rather than purified water sources. However, use of such water sources lead to difficulties in dispersing the high molecular weight polymers to use concentrations. Inversion of w/o lattices in such water sources can result in slow inversion times and/or require multistage dilution and mixing procedures; it can also result in coagulation, precipitation, or gross phase separation of polymer upon or after contact of the latex with the diluted water environment. The products of such instability cause plugged equipment in the field and failure to accomplish mobility control within the reservoir. These problems remain largely unaddressed by conventional methods and equipment developed for inversion of w/o lattices in the field. Thus there is a need to address inversion of w/o lattices in field conditions where the use water source has high total dissolved solids, is present at high temperature, or both.
U.S. Pat. No. 8,383,560 describes a two-step inversion apparatus that is designed to take advantage of the concentration effect with latex polymer inversion. In the first step, a w/o polymer latex is diluted to yield a polymer solution having about 5000 ppm to 20,000 ppm polymer solids employing a first static mixer having a pressure drop of at least 2 bars between the inlet and outlet thereof. The shear associated with the pressure drop facilitates the dispersal of the w/o latex into fine droplets in the water. These droplets, with the aid of surfactants, then proceed to release the polymer particles into the water. In the second step after sufficient residence time, the partially diluted latex is combined with a second stream of water and applied to a second static mixer having a pressure drop of at least 1 bar between the inlet and outlet. This results in a polymer solution having between 500 and 3000 ppm, in practice between 1000 and 2000 ppm polymer solids. However, such two-stage inversion apparatuses still require a relatively large equipment footprint.
Thus, there is a need in the industry to develop devices for use in EOR applications wherein w/o latex inversion is carried out under conditions of restricted space and/or equipment weight allowances. There is a need in the industry to provide devices for accomplishing w/o latex inversion in a single step. There is a need in the industry for inversion equipment capable of enabling continuous inversion and hydration, in a total time of 180 minutes or less. There is a need in the industry to accomplish a single-step inversion process under harsh conditions such as use of water sources having high temperature, high total dissolved solids, or both.